The present invention relates to an objective lens actuator used in an optical disk device for recording and reproducing information on/from an optical disk such as a magneto-optical disk, for displacing an objective lens in a focus direction and in a track direction so as to perform focus servo and tracking servo to the optical disk.
FIGS. 9(a) through 9(c) show a schematic structure of a magneto-optical disk device 51, one of optical disk devices. The magneto-optical disk device 51 is made up of a mechanical chassis 52, a spindle motor 53 fixed to the mechanical chassis 52, an optical pick-up 54 provided with an objective lens 59, a magnetic head unit 55 fixed to the optical pick-up 54, a feeder 57 and guide axes 58 for moving the optical pick-up 54 in a direction of a radius of a magneto-optical disk 56 chucked by the spindle motor 53.
The optical pick-up 54 focuses a light beam emitted from a laser light source (not shown) onto the magneto-optical disk 56 by the objective lens 59. The objective lens 59 is driven in a focus direction and in a tracking direction by an actuator 60 which will be mentioned later [see FIGS. 10(a) and 10(b)], so as to follow the surface vibration of a disk and the decentering of a track.
The magnetic head unit 55 is made up of a suspension 55a and a slider section 55b. The suspension 55a presses the slider section 55b with an appropriate force by elastic deformation, so as to prevent the slider section 55b from detaching from the magneto-optical disk 56 when the surface vibration of the disk occurs.
FIGS. 10(a) and 10(b) show a detailed structure of the objective lens actuator 60. The objective lens 59 is held by an objective lens holder 61. The objective lens holder 61 is fixed to an actuator base 63 via four support wires 62 which are located in parallel to one another. More specifically, the objective lens holder 61 is fixed via the support wires 62 to a rising section 63b of the actuator base 63, which is formed by raising one end of a horizontal section 63a of the actuator base 63 in a lengthwise direction, vertically with respect to the horizontal section 63a. 
On a side of the objective lens holder 61 opposite to a side facing the rising section 63b, a focus coil 64 and a tracking coil 65 are fixed. A magnetic flux is generated in a cavity in a magnetic circuit 66 made up of permanent magnets 66a and a yoke 66b, and the magnetic flux and a current flowing the foregoing coils react one another, permitting the objective lens 59 to be displaced freely in the focus direction and the tracking direction.
When a light beam is spot-emitted on the magneto-optical disk 56 by the objective lens 59 of the optical pick-up 54, as shown in FIG. 11, the temperature of a section subjected to the spot emission in a recording medium 56b formed on a disk substrate 56a of the magneto-optical disk 56 is increased, and the coercive force of a magnetic substance of the recording medium 56b in the section is decreased. Here, if a magnetic field is given to the section subjected to the spot emission by a magnetic head 55c of the slider section 55b, the section is easily magnetized, and information is recorded on the magneto-optical disk 56.
Incidentally, in the magneto-optical disk device 51 structured as mentioned above, when trying to displace the objective lens holder 61 in the focus direction [in a direction of an arrow A in FIG. 10(b)] so as to perform focus servo, a reaction force [a force acting in a direction of an arrow B in FIG. 10(b)] is applied to the magnetic circuit 66 and the actuator base 63 which directly supports the magnetic circuit 66, vibrating the whole optical pick-up 54. The vibration is transmitted to the magneto-optical disk 56 via the guide axes 58 and the spindle motor 53. As a result, the magneto-optical disk 56 is vibrated, which makes it difficult to perform stable focus servo.
Thus, for example, Japanese Unexamined Patent Publication No. 7-105550 (Tokukaihei 7-105550, published on Apr. 21, 1995) (U.S. Pat. No. 5,719,834) discloses a structure for elastically supporting the magnetic circuit 66 with flexibility in a focus direction, using two parallel leaf springs 67, as shown in FIGS. 12(a) and 12(b). More specifically, the magnetic circuit 66 is fixed, via the parallel leaf springs 67 located parallel to the horizontal section 63a of the actuator base 63, to a rising section 63c of the actuator base 63, which is formed by raising the other end of the horizontal section 63a in a lengthwise direction (on a side opposite to the rising section 63b in the lengthwise direction), vertically with respect to the horizontal section 63a. Therefore, in this structure, a slight vanity is formed between the magnetic circuit 66 and the horizontal section 63a. 
In this structure, when the objective lens holder 61 is moved in the focus direction, the magnetic circuit 66 is moved in a direction opposite to the moving direction of the objective lens holder 61, by a reaction force applied to the magnetic circuit 66. At this time, the parallel leaf springs 67 supporting the magnetic circuit 66 flex, and eventually, the vibration of the magnetic circuit 66 caused by the reaction force is absorbed by the flexure of the parallel leaf springs 67. Therefore, this structure can prevent the actuator base 63 and the optical pick-up 54 from being vibrated by the displacement of the magnetic circuit 66 in accordance with the displacement of the objective lens holder 61 in the focus direction, achieving stable focus servo.
However, in the structure disclosed in the foregoing publication which supports the magnetic circuit 66 using the parallel leaf springs 67, there is a problem that it is difficult to downsize the actuator 60.
That is, in the structure disclosed in the foregoing publication, since the parallel leaf springs 67 are provided between the magnetic circuit 66 and the rising section 63c, a length a of the horizontal section 63a in the lengthwise direction, is increased compared with the case where the parallel leaf springs 67 are not provided, resulting in an increase in the size of the actuator 60.
Here, it can be considered, for example, to adopt a technique for downsizing the actuator 60 by decreasing the length of the parallel leaf springs 67 in the lengthwise direction without changing a spring constant, using a material with a low Young""s modulus to form the parallel leaf springs 67. In this case, however, the parallel leaf springs 67 flex more, compared with the case where longer parallel leaf springs 67 having the identical spring constant are formed using a material with a higher Young""s modulus, and the parallel leaf springs 67 are likely to be subjected to plastic deformation. This structure causes a problem in the displacement of the magnetic circuit 66 in accordance with the displacement of the objective lens holder 61 in the focus direction.
Incidentally, in order to prevent plastic deformation, it can also be considered, for example, to provide a stopper for limiting the displacement of the parallel leaf springs 67. In this case, however, if the length of the parallel leaf springs 67 is short, the positioning accuracy of the stopper should be enhanced, increasing design load.
The present invention is made to solve the foregoing problems, and its object is to provide an objective lens actuator which is capable of performing stable focus servo even if a reaction force of a force acting on an objective lens holder is generated, and being downsized with a simple structure.
To achieve the foregoing object, an objective lens actuator in accordance with the present invention is structured so as to include:
an objective lens holding element for holding an objective lens which focuses a light beam onto an optical disk;
an action force generation source for generating an action force for displacing the objective lens holding element in a focus direction of the optical disk, by using a coil and a magnetic circuit;
a support member for supporting either the coil or the magnetic circuit as a supported element, with respect to an actuator base,
wherein, when the action force generation source generates the action force, the support member supports the supported element in such a manner that the supported element can be rotated about an arbitrary point of the support member by a reaction force of the action force; and
an equation a=I/Mh holds,
where M is a mass of the supported element, I is a moment of inertia of the supported element about a center of gravity, a is a distance between the center of gravity of the supported element and a line of action in a direction of the action force, and h is a distance between a center of rotation in the support member and the center of gravity of the supported element.
According to the foregoing structure, when the objective lens holder is displaced in the focus direction by the action force generated by the action force generation source, among the coil and the magnetic circuit in the action force generation source, either one of them which is supported by the support member comes to rotate about the arbitrary point of the support member.
Here, since the equation a=I/Mh is satisfied, even if the action force is an active force such as a sinusoidal wave excitation force, the support member deforms in the same way as in the case where a static force is applied, and thus the supported element rotates statically. With this structure, the reaction force can be surely absorbed. That is, even if the supported element is excited by the reaction force, the excitation is surely retrained by the rotation of the supported element supported by the support member. As a result, the structure eliminates the vibration of the actuator base by the reaction force transmitted via the support member, and the vibration of the optical disk via the actuator base. Therefore, according to the foregoing structure, it becomes possible to eliminate the vibration of the optical disk caused by the reaction force, and to perform stable focus servo.
In the foregoing structure, since the reaction force is absorbed by the rotation of the supported element, assuming that the support member is, for example, a leaf spring, even when the length of the support member is shortened compared with that used in the conventional structure having parallel leaf springs, while the spring constant, the geometrical moment of inertia, and the Young""s modulus of the leaf spring remain the same as those of the leaf spring used in the conventional structure, the effect of reaction force absorption, that is, the effect of vibration absorption can be surely obtained. That is, according to the foregoing structure, the length of the support member can be shortened without deteriorating the vibration absorption effect. As a result, the actuator can be surely downsized compared with the conventional parallel leaf springs structure.
Besides, since the length of the support member can be shortened even though it is not made of a material having a low Young""s modulus, there is no need to worry about plastic deformation of the support member. Therefore, there is no need to additionally provide a structure for preventing the plastic deformation, and thus the support member can be shortened with a simple structure.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.